How to render specular highlights as a separate image with alpha channel?
You should add in the specular with color, not mix shaders. In that way you will get a white highlight. 

Here I'm using color ramps to make a mask for transparency and removing some of the gray on the specular before adding it to a solid color.



You can also render out Diffuse and Glossy and use the Compositor to choose color.



You need to add light passes for gloss direct and diffuse direct


The new passes can be then be exported from the Compositor via Image editor (set the image to be "Viewer Node") and then saved from there Image->Save As...